1. Field of the Invention
The present invention relates to a slit lamp microscope, and more particularly to a slit lamp microscope in which an illumination optical system is improved so that not only a slit image of a specific part, to be observed, of an eye to be medically examined but also a peripheral portion of the part to be observed may be clearly observed.
2. Description of the Related Art
A slit lamp microscope is used as a kind of microscopes for the use in an ophthalmic field. This is used by an ophthalmologist for his or her routine medical examination. A variety of lesion portions of the eye to be examined may be observed through the microscope by devising the illumination method.
As shown in FIG. 9 in concept, in this slit lamp microscope, a ray of light from a light source 201 is formed into a slit-like beam (slit beam) and projected to a cornea Ed of the eye E to be examined so that the cornea Ed is brought into an optically cut condition, and a reflected ray from the corneal portion which has been optically cut is introduced into an observation optical system (not shown) and an image processing system (not shown) through an objective lens 202. As shown in FIG. 10, the corneal sectional image Ed' is displayed on a screen of an image monitor 203. The slit lamp microscope may be used to observe a slight lesion portion of the eye E to be examined by utilizing the Tyndall effect (see "SLIT LAMP" by Motoichi Itoi and Sadao Kanakami, p25, FIG. 34, published by Medical Aoi Publication Co.).
However, in case of such a conventional slit lamp microscope, for example, the corneal cross sectional image Ed' may be clearly observed but the tissue (sclera, iris or the like) except for the corneal cross section cannot be observed at all. This is because the cornea is projected by the slit-like illumination light so that the illumination field is very narrow and the peripheral region except for the corneal cross section becomes so dark (indicated by the cross-hatchings in FIG. 10) so that the region can not be be imaged.
However, if the tissue condition of the peripheral portion around the specific portion of the eye E to be examined is imaged and a position and a size of the lesion portion over the entire eye to be examined may be known, it is possible to perform a suitable examination. Accordingly, it is highly demanded to make it possible to observe the tissue condition of the peripheral portion around some specific portion.
If this is met, it is also advantageous to facilitate the explanation of the lesion portion to a third party except for the patient (i.e., family members of the patient, the persons having to do with some symposium or special magazine or the like).
Under such circumstances, conventionally, a means for projecting another ray of illumination light for illuminating the overall eye to be examined is provided separately from the ray of the slit-like illumination light. The eye to be examined is simultaneously projected by the slit-like illumination light and the other illumination light to thereby image the entire image of the eye to be examined.
However, in the case where the illumination light for the overall eye to be examined is added, the contrast of the image of the lesion portion by the projection of the slit-like illumination light is degraded, resulting in degradation of the image quality. Accordingly, it is troublesome to make a clear examination to the lesion portion of the eye to be examined.